Java8 Map集合中put()与putIfAbsent()的区别

Map集合中put与putIfAbsent的区别

put方法：

V put(K key, V value);

putIfAbsent方法：

V putIfAbsent(K key, V value)；

这两种方法都是以key-value键值对的形式存在到map集合中，那么它们两个有什么区别呢？

我们可以从map官网注释中看出：

1.使用put方法添加键值对，如果map集合中没有该key对应的值，则直接添加，并返回null，如果已经存在对应的值，则会覆盖旧值，value为新的值。

2.使用putIfAbsent方法添加键值对，如果map集合中没有该key对应的值，则直接添加，并返回null，如果已经存在对应的值，则依旧为原来的值。

    /**
     * Associates the specified value with the specified key in this map
     * (optional operation).  If the map previously contained a mapping for
     * the key, the old value is replaced by the specified value.  (A map
     * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
     * if {@link #containsKey(Object) m.containsKey(k)} would return
     * <tt>true</tt>.)
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>,
     *         if the implementation supports <tt>null</tt> values.)
     * @throws UnsupportedOperationException if the <tt>put</tt> operation
     *         is not supported by this map
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     * @throws NullPointerException if the specified key or value is null
     *         and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     */
    V put(K key, V value);

   
    /**
     * If the specified key is not already associated with a value (or is mapped
     * to {@code null}) associates it with the given value and returns
     * {@code null}, else returns the current value.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code
     * map}:
     *
     * <pre> {@code
     * V v = map.get(key);
     * if (v == null)
     *     v = map.put(key, value);
     *
     * return v;
     * }</pre>
     *
     * <p>The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-
     *         restrictions">optional</a>)
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-
     *         restrictions">optional</a>)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-
     *         restrictions">optional</a>)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (<a href="{@docRoot}/java/util/Collection.html#optional-
     *         restrictions">optional</a>)
     * @since 1.8
     */
    default V putIfAbsent(K key, V value) {
        V v = get(key);
        if (v == null) {
            v = put(key, value);
        }

        return v;
    }

example：

Map<String, String> map = new HashMap<>();
System.out.println(map.put("1", "4"));
System.out.println(map.get("1"));

System.out.println(map.put("1", "5"));
System.out.println(map.get("1"));

System.out.println(map.putIfAbsent("1", "6"));
System.out.println(map.get("1"));

System.out.println(map.putIfAbsent("2","1"));
System.out.println(map.get("2"));

返回结果：

null
4
4
5
5
5
null
1